I didn't see this in the guide, so I assume this is not the case. I just wanted to make sure.

Do the genders of the Pokémon have any affect on game play other than breeding? Some friends of mine insist that males have higher attack and speed and females have higher defense and HP. Any truth to this?

In Generation II, IVs were put to use in another manner besides determining stats. While this was, of course, their major function, other things determined by the IVs of a Pokémon included the type and power of Hidden Power, a Pokémon's gender (if it had one), and whether or not it was a shiny Pokémon. As in Generation I, each IV is randomly picked between 0 and 15 at the time of encounter. While there are six stats, instead of the five there were in Generation I, both Special Attack and Special Defense are determined by one IV for Special, facilitating compatibility with Generation I. IVs can be determined by the player in the same way they could be during Generation I.

Gender was determined by IVs in Generation II, more specifically by the Attack IV of the Pokémon in relation to its maximum value, with the higher IVs making for a male Pokémon, and lower ones making for a female. A female Typhlosion, whose species has a gender ratio of 7?:1?, would have only 0 or 1 as her Attack IV, while a female Golduck, whose species has a gender ratio of 1?:1?, would have an Attack IV of any value from 0-7. Males of both would be designated for the other IVs.

Does that effect attack stat only or special attack as well? And is that only in GSC, 'cause in my Platinum my female Blaziken is weaker than all my male Pokemon but has a higher attack stat than all of them.

It affects Attack stat only. Also it is only GSC because they had to use the DV system to determine shiny, gender etc in order to have compatibility with RGBY. Later on they used the personality value which was separate and was NOT compatible with earlier games.